I agree Regent should have done more.
Having said that, the difference between a shuttle bus and a taxi is about $100. No matter what, your vacation and your health and not worth impacting over that small amount given the amount you spent on the cruise. Yes, Regent should have done more. Yes, if I were the TA I would pay it myself. But at the end of the day, you don't let $100 raise your BP or ruin your trip.